Cycles high angle face artefact
Shadow Terminator Offset
you can find this setting in
Properties ‣ Object Properties ‣ Shading
increase it to 0.15 which is really enough
Note: this is a new feature came in blender 2.90 so you need to update blender to some newer version



Note
This property artificially alters the scene’s lighting and is not
energy conserving and consequently not physically accurate.

Got from the blender Manual: https://docs.blender.org/manual/en/2.90/render/cycles/object_settings/object_data.html#shading